Finally, and most important of all: KNOW YOURSELF.
What are you getting from the relationship? Are you actually a masochist? A codependent perhaps? Why is this relationship attractive and interesting?
Define for yourself what good and beneficial things you believe you are receiving in this relationship.
Define the things that you find harmful TO YOU. Develop strategies to minimize the harm to yourself. Don't expect that you will cognitively be able to reason with the narcissist to change who they are. You may have some limited success in getting your narcissist to tone down on the really harmful behaviours THAT AFFECT YOU.
This can only be accomplished in a very trusting, frank and open relationship.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 2 DVDs with 12 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)